A US military analyst, Bradley Manning, has been arrested on suspicion of leaking classified combat video and documents to a whistle-blower website.
Specialist (Spc) Manning, 22, from Potomac, Maryland, is being held pending further investigations.
A former hacker said he turned Mr Manning in out of concern for US national security.
The WikiLeaks website posted a video which it says shows the US military shooting civilians in Baghdad in 2007.
WikiLeaks has not confirmed Spc Manning as its source for the helicopter footage.
News of Spc Manning's arrest first broke on the Wired.com website.
